LfRI^ART 

 New YORK 

 BOTANICAL 



GARDEN. 



Transpiration and the Ascent of Sap. 



By 

 Henry H. Dixon, Sc.D., F.R.S. 



Prof, of Botany in the University of Dublin. 



The water of the transpiration stream enters at the roots, passes 

 up the stem and is given oif from the leaves. It will be convenient 

 to discuss the processes taking' place in each of these organs separately 

 so far as they effect the stream and then consider how these processes 

 are correlated. 



In ^regard to the exhalation of water vapour from the leaves 

 early experimenters liave shown that cuticular transpiration is usually 

 insignificant compared with diastomatic diffusion. The efficiency of 

 diastomatic transpiration was first clearly explained by Brown and 

 Escombe.^) 



The minute cross-sections of the openings of stomata and the 

 comparatively large area occupied by the practically impermeable 

 cuticle made it difficult to understand how the observed quantities 

 of water escape from the leaf. These authors showed, however, that 

 an unexpected law governs the diffusion of water vapour through 

 a number of minute perforations in an impermeable membrane. 

 According to this law it follows that the amount which diffuses 

 through the perforations is not only, as one would on first thoughts 

 expect, proportional to the sum of their areas, but may vastly exceed 

 this proportionality; and, consequently, the diffusion through a number 

 of minute pores, like the stomata, will be much greater than through 

 one large aperture having a cross-section equal to the sum of the 

 areas of the stomata. 



') Brown and E s c o m b e , Static Diffusion of Gases and Liquids in Relation 

 to the Assimilation of Carbon and Translocation in Plants. Phil. Trans. Roy. See. 

 Lond. B., Vol. 193, pp. 223—292, 1900, of which an abstract appeared in the Proc. 

 ^Roy. Soc, Vol. 66 and in Ann. of Bot., Vol. XIV, Sept. 1900, pp. 537—542. - 



^^ Progressas rei botanicae III. ^ 



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